Did you evaluate using something like Redis for this? It's got an atomic increment command that guarantee's unique ID's and the performance is stupid fast.
We love Redis at Instagram (it powers a lot of our systems...more write-ups on these soon), and considered it for our ID generation, but it would have introduced a single point of failure, unless we split the load between several Redis instances, at which point it would be hard to make the IDs time-sortable. Also, most of our Redis systems are durable within a minute (we write to disk on a slave every minute), but if we were to lose the master and slave simultaneously (imagine an EC2 network issue or such), then it would be hard to know what the last known 'good' ID was--not an insurmountable problem, but one more moving part to worry about. That said, using Redis with something like the upcoming Redis Cluster could be a good choice, though.
